1. Field of the Invention
The present invention relates to a semiconductor integrated circuit device that can reduce dynamic noise generated in power supply lines and prevent a malfunction operation thereof.
2. Description of the Related Art
In a recent semiconductor integrated circuit device, power consumption per unit area increases accompanying an increase in integration of the device, thus causing a high noise level in the power supply therefore. Furthermore, the power supply voltage for supplying to the semiconductor integrated device decreases accompanying the miniaturization of the device, resulting in a decrease in an ability of the device to resist the power supply noise. For the purpose of eliminating these problems, decoupling capacitor cells are provided on a PCB (Printed Circuit Board) for coupling between the semiconductor integrated circuit devices (described in, for example, “Japanese Patent Kokai No. H10-12825”). Capacitor cells are also provided in unused regions of the semiconductor integrated circuit not occupied by the decoupling capacitor cells and logic cells. The decoupling capacitor cells and capacitor cells are used for suppressing the power supply noise.
A conventional semiconductor integrated circuit will now be described with reference to the accompanying drawings. FIG. 1 illustrates a plan view of a conventional semiconductor integrated circuit device 68 described in, for example, “Japanese Patent Koki No. 2004-55874”. As shown in FIG. 1, a power supply voltage is supplied from an outer circumference of the semiconductor integrated circuit device 68, so that a voltage drop 70 of the power supply increases as it approaching the center of the semiconductor integrated circuit device 69. A voltage drop margin 72 with respect to a permissible voltage 71 decreases as it approaching the center of the semiconductor integrated circuit device 69. The logic cells can normally operate above the permissible voltage 71. Thus, the conventional semiconductor integrated circuit device is provided with capacitor cells, each of which has a charge storage capacity sufficient to compensate the voltage drop.
However, the following problems occur in the conventional semiconductor integrated device described above. As shown in FIG. 2, a method for designing the device includes the steps of forming the logic cells (S21), forming the capacitor cells (S22), and forming empty cells (S23). After forming the logic cells (S21), the capacitor cells are formed on the semiconductor integrated circuit device (step S22). The capacitor cells are formed in spaces between the logic cells. Thus, the capacitor cells are densely distributed in regions where the logic cells are thinly distributed, whereas the capacitor cells are thinly distributed in regions where the logic cells are densely distributed. FIG. 3 illustrates a plan view of a conventional semiconductor device which is designed by using the method shown in FIG. 2. A sufficient number of capacitor cells are not disposed adjacent to regions 67 where logic cells 66 are densely disposed. The logic cells generate noise which is superimposed on the power supply voltage, so that the noise is transmitted through power supply lines 61 and ground lines 62. FIG. 4 is a schematic circuit diagram of a part of the semiconductor integrated circuit of FIG. 3. As shown in FIG. 4, the logic cells 66 are substantially separated from the capacitor cells 65. The power supply noise generated by the logic cells 66 are transmitted a long distance to the capacitor cells 65, so that parasitic resistances of the power supply lines 61 and ground lines 62 increases. As a result, there arises a problem that the decoupling effect of the capacitor cells decreases and thus the power supply noise is not effectively reduced.
The capacitor cells are provided in spaces not occupied by the logic cells. An area of the capacitor cell is adjusted to that of the spaces. In the case where the space has a small area, capacitor per area of the capacitor cell can not be increased. Therefore, in the case where the spaces between the logic cells are small in area, capacitor cells that have small capacities cannot be provided in the spaces. Thus, efficient decoupling capacities can not be obtained in the semiconductor integrated device and thus the power supply noise is not effectively reduced.
In addition, in the case where the capacitor cells are provided locally in the center of the semiconductor integrated circuit device from the view of voltage drops in the power supply lines of the center of the device, the power supply noise of the semiconductor integrated device can not be decreased sufficiently. Moreover, in the case where the logic cells are formed prior to the capacitor cells, the semiconductor integrated circuit is so configured that the logic cells are locally distributed in the center of the device. Unnecessary delays occur in the integrated circuit, thus decreasing operation speed of the integrated circuit.